Tried to solve the following problem the other day on my standby database:

File #7 added to control file as 'UNNAMED00007' becausethe parameter STANDBY_FILE_MANAGEMENT is set to MANUALThe file should be manually created to continue.MRP0: Background Media Recovery terminated with error 1274Fri Apr 8 10:00:22 2005Some recovered datafiles maybe left media fuzzyMedia recovery may continue but open resetlogs may fail

I had done this before, but - couldn't remember how to do it. The same problem can occur when you are recovering your database and a data file has been added to it which was not part of your backup set.

So, here is the solution! I came close to it while looking up the Oracle SQL reference:

On the standby, use "ALTER DATABASE CREATE DATAFILE" as shown in this example: